Ribosomal protein S6 is phosphorylated in HeLa cells; the level of phosphorylation changes dramatically when stationary phase cultures renew their growth in response to fresh medium or serum. The nature and function of protein S6 phosphorylation will be studied in detail. The intramolecular sites of phosphorylation will be identified. Molecules that may interact with S6 in each state of phosphorylation will be identified by the use of cross-linking reagents. Possible functions of S6 in controlling initiation frequency, Messenger RNA selection, or ribosomal RNA synthesis will be tested.